A plastic lens is light in weight, has excellent toughness, and can easily be dyed. Properties that are particularly required for a plastic lens include low density, high transparency, reduced yellowness, optical properties such as high refractive index, high Abbe's number, high heat resistance and high strength. A high refractive index allows thinning of the lens while a high Abbe's number reduces chromatic aberration of the lens.
Recently, a number of examples using an organic compound having a sulfur atom have been reported for the purpose of achieving a high refractive index and a high Abbe's number. Among them, a polyepisulfide compound having a sulfur atom is known to have good balance between the refractive index and the Abbe's number (Patent document 1). Moreover, since polyepisulfide compounds can react with various compounds, their compositions with various compounds such as thiourethane have been proposed for enhancing physical properties (Patent documents 2-5). In addition, as a composition comprising a polyepisulfide compound, an optical material containing an inorganic compound having a sulfur atom and/or a selenium atom has been proposed with the aim of achieving a higher refractive index (Patent document 6).